Dual Gate Layout for Thin Film Transistor

ABSTRACT

A dual gate layout of a thin film transistor of liquid crystal display to alleviate dark current leakage is disclosed. The layout comprises (1) a polysilicon on a substrate having a L-shaped or a snake shaped from top-view, which has a heavily doped source region, a first lightly doped region, a first gate channel, a second lightly doped region, a second gate channel, a third lightly doped region and a heavily doped drain region formed in order therein; (2) a gate oxide layer formed on the polysilicon layer and the substrate, (3) a gate metal layer then formed on the gate oxide layer having a scanning line and an extension portion with a L-shaped or an I-shaped. The gate metal intersects with the polysilicon layer thereto define the forgoing gate channels. Among of gate channels, at least one is along the signal line, which is connected to the source region through a source contact.

RELATED APPLICATION

This application is a divisional application of prior application Ser.No. 11/211,606, which is filed on Aug. 26, 2005, as a divisionalapplication of prior application Ser. No. 10/624,479, filed on Jul. 23,2003, issued as U.S. Pat. No. 6,936,848 and are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a dual gate structure of thin filmtransistor for liquid crystal display (LCD), and more particularly to adual gate with at least one gate channel gat along data line for lowtemperature polysilicon LTP LCDs so as to inhibit leakage current andprovide picture resolution improvement process windows.

BACKGROUND OF THE INVENTION

A liquid crystal display (LCD) is a flat display with properties of lowpower consumption, low significantly, space occupation and weight incomparison with a conventional cathode ray tube (CRT) and without curvesurface as a CRT display has. Hence, the liquid crystal display haswidely been applied in all sorts of merchandises, including consumptiveelectronic products, such as pocket calculators, electronicdictionaries, watches, mobile phones, portable notebooks, communicationterminals, display panels, desk-top personal computers, and even highdpi (dots per inch) television (HDTV) and so on. The most populardisplay is an active-type thin film transistor liquid crystal displays(TFT-LCD) due to the fact that the viewing angle, response time and thecontrast performance are much better than that passive type LCD.

For a long term, amorphous silicon is a main material for TFTmanufacture in TFT-LCD device. However, nowadays, another choice isprovided, forming transistor using polysilicon is found superior toamorphous silicon. The low temperature polysilicon type TFT-LCD(LPTFT-LCD) may even become a main stream due to the better performanceof carrier mobility in polysilicon than in amorphous. Another advantageof LPTFT-LCD had is the driving circuit can be formed simultaneouslywith the pixel TFT fabrication Therefore, LPTFT-LCD can provide a fasterswitched speed than other types LCD.

Certainly, LTP TFT-LCD has some drawbacks need to be overcome. Forinstance, the device usually has a rather large leakage current duringTFT turn off. To overcome this defect, Inoue et al, proposed a conceptof dual gate TFT structure to inhibit the problem of leakage current.Please refer to the reference, U.S. Pat. No. 5,693,959. Another methodis provided by Ha et al in U.S. Pat. No. 5,940,151; the patent provideslightly doped drain (LDD) technique to alleviate the leakage currentproblem.

The present invention concentrates to the dual gate structure of TFT.Please refer to FIG. 1A, a top view, and the cross-sectional view inFIG. 1B, which is viewed along cut-line a-a′ of FIG. 1 A. The numeral909 is denoted a polysilicon layer, which is formed to constitute aheavily doped source region 909 a, a lightly doped source region 909 b,a first channel 909 c, a lightly doped region 909 d, which is in betweendual gate, a second channel 909 d, a second channel 909 e, a lightlydoped drain region 909 f, and a heavily doped drain region 909 g. Thescan line 903 includes dual gates, one over the first channel 909 c andthe other over the second channel 909 e. The signal line 904 usuallymade of aluminum is contacted to the heavily doped source region 909 athrough source contact 910. The drain metal line is connected to atransparent conductive electrode by mean of through-hole 913 and draincontact 911 to the heavily doped drain region 909 g.

The forgoing dual gate provided two gates in parallel and formed alongthe scan line. Unfortunately, in the design of color filter layout, theprimary colors: red, blue, and green are usually along scan line. As aresult, the resolution of display will suffer some limits. Since, thetotal lengths of two channel 909 c, 909 e and the interval in between909 d will be restricted owe to the constraints of lithographic machinefor TFT. Since the problem of the pixel is crowded along scan linedirection. However, there is no such problem along the data line. Threesub-pixels for three primary colors are not along this direction. Bycontrast, it provides more process windows. Subsequently, an object ofthe present invention is to provide a method, which is to reshuffle thepositions of the dual gate. Some or part of the loadings of dual gate onthe scanning line is shared by the signal line and thus solve theforgoing problem.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a TFT with adual gate structure for a low temperature polysilicon LCD to inhibitcurrent leakage.

The second object of the present invention is to improve the issue ofabout conventional dual gate structure, which both of the gates arealong scanning line that inferior to the high resolution.

Five preferred embodiment are disclosed. In the first preferredembodiment, a polycrystalline silicon having a stair shaped in geometryfrom top view formed on a substrate for a source region, a first dopedregion, a first gate channel, a second doped region, a second gatechannel, a third doped region, and a drain region formation. Thepolycrystalline silicon layer having a vertical section and a horizontalsection intersect, respectively, with a scanning line, and an I-shapedextension portion thereof to form a first gate channel and a second gatechannel.

In the second preferred embodiment, the polycrystalline silicon having aL-shaped in geometry from top view formed on a substrate for a sourceregion at an end of horizontal section, a first doped region, a firstgate channel, a second doped region, a second gate channel, a thirddoped region, and a drain region at an end of vertical sectionformation. The polycrystalline silicon layer having a vertical portionintersects with a scanning line, and a horizontal portion of L-shapedextension portion to form a first gate channel and a second gatechannel.

In the third preferred embodiment, the polycrystalline silicon having aL-shaped in geometry from top view formed on a substrate for a sourceregion at an end of horizontal section, a first doped region, a firstgate channel, a second doped region, a second gate channel, a thirddoped region, and a drain region at an end of vertical sectionformation. The polycrystalline silicon layer having a horizontal sectionand a vertical section intersect, respectively, with an I-shapedextension portion and scanning line to form a first gate channel and asecond gate channel.

In the fourth preferred embodiment, the polycrystalline silicon having aL-shaped in geometry from top view formed on a substrate for a sourceregion at an end of horizontal section, a first doped region, a firstgate channel, a second doped region, a second gate channel, a thirddoped region, and a drain region at an end of vertical sectionformation. The polycrystalline silicon whose vertical section intersectswith a scanning line and a horizontal portion of L-mirror shapedextension portion to form a first gate channel and a second gatechannel.

In the fifth preferred embodiment, the polycrystalline silicon having aL-shaped in geometry from top view formed on a substrate for a sourceregion at an end of horizontal section, a first doped region, a firstgate channel, a second doped region, a second gate channel, a thirddoped region, and a drain region at an end of vertical sectionformation. The polycrystalline silicon layer having a horizontal sectionand a vertical section intersect, respectively, with a vertical sectionand a horizontal section of a LI-shaped extension portion to form afirst gate channel and a second gate channel.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1A is a plan view illustrating a dual gate structure of a TFT-LCDin accordance with the current invention.

FIG. 1B is a cross-sectional view illustrating a dual gate structure ofa TFT-LCD in accordance with the current invention.

FIG. 2A is a plan view illustrating a dual gate structure of a TFT-LCDin accordance with the first preferred embodiment of the presentinvention.

FIG. 2B is a cross-sectional view illustrating a dual gate structure ofa TFT-LCD in accordance with the first preferred embodiment of thepresent invention.

FIG. 3 is a plan view illustrating a dual gate structure of a TFT-LCD inaccordance with the second preferred embodiment of the presentinvention.

FIG. 4 is a plan view illustrating a dual gate structure of a TFT-LCD inaccordance with the third preferred embodiment of the present invention.

FIG. 5 is a plan view illustrating a dual gate structure of a TFT-LCD inaccordance with the fourth preferred embodiment of the presentinvention.

FIG. 6 is a plan view illustrating a dual gate structure of a TFT-LCD inaccordance with the sixth preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In addition to large scales is a generic trend for LCD monitors or LCDtelevision, another trend is to raise the resolutions so as to improvethe picture quality. Due to the dark leakage current problem, dual gateis a common approaching to solve the leakage current of LTP TFT LCD.However, conventional dual gate structure is detrimental to theresolution improvement because the dual gates are common formed alongthe scanning line. The structure of the present invention disclosed canresolve such problem.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and is provided in the contextof a patent application and its requirement. Various modifications tothe preferred embodiment and generic principles and features describedherein will be readily apparent to those skilled in the art. Thus, thepresent invention is not intended to be limited to the embodiment shownbut is to be accorded the widest scope consistent with the principlesand features described herein. For example, the present invention willbe described in the context of using n-type conductive impurities whichis just for illustrating convenience, one of ordinary skill in the artwill readily recognize that p-type impurities could be here. And its usewould be within the sprit and scope of the present invention.

The present invention proposed several embodiments and will be describedas follows:

According to the first preferred embodiment, referring to FIG. 2A, a topview for one pixel layout. The polycrystalline silicon layer 100 formedon the glass substrate presents as a stair-shaped having regionsincluding 100A, 100B,100C, 100D, 100E, 100F, 100G, 100H, and 100I. Thestair-shaped polycrystalline silicon layer 100 intersects with thescanning line 120 and its I-shaped extension portion by two regions. Oneis formed as the first channel 100C, and the other is the second channel100G. Two regions 100B, 100D abutting both sides of the first channel100C are n-type lightly doped regions. Side regions 100F, 100H abuttingthe second channel 100G are n-type lightly doped regions 100F 100H too.To further reduce the resistance between channel regions 100C and 100G,one n+ heavily doped region 100E is usually included. Still, the n+heavily doped source region 100A is contacted with the data line 130through contact windows 132. The data line 130 is made of metal such asaluminum. The n+ heavily doped drain region 100I is contacted with thestorage capacitor (not shown) and connected to the first interconnectline 150, which is formed on the first inter-level dielectric layer 180through contact windows 133 and further through a via 162 to connect thetransparent conductive electrode 160, which is formed on the secondinter-level dielectric layer 190.

The geometry of regions 100D, 100E, and 100F presented as right angledistributed, as is shown in the figure. It is not intended to limit theclaim scope. It can be other geometry, for example they can be presentedas arc-shaped or a shortest line between the channels 100 C and 100G.

The second preferred embodiment is shown in FIG. 3, a top view. Thepolycrystalline silicon layer 200 formed on the glass substrate presentsas a L-shaped having regions including 200A, 200B,200C, 200D, 200E,200F, and 200G. The L-shaped polycrystalline silicon layer 200intersects with the scanning line 220 and its L-shaped extension portionby two regions. One is formed as the first channel 200C, and the otheris the second channel 200E. As aforementioned in the first preferredembodiment, an insulating layer is formed before forming gate metallayer 220. the regions abutting both sides of the first channel 200C andthe second channel 200E are n-type lightly doped regions 200B, 200D and200D, 200F, respectively. Still, the n+ heavily doped source region 200Ais contacted with the metal data line 230 through contact windows 232.The n+ heavily doped drain region 200G is contacted with the storagecapacitor (not shown), the first interconnect line (not shown) and thetransparent conductive electrode 260. The detailed descriptions are asfirst preferred embodiment.

Worthwhile if the channel 200C does not distant from the channel 200E,one n-lightly doped region 200D formed is enough. However, the n-lightlydoped region 200D can also have a heavily doped region 200D′ in it toreduce resistance.

The third preferred embodiment is shown in FIG. 4, a top view. Thepolycrystalline silicon layer 300 formed on the glass substrate presentsas a L-shaped having regions including 300A, 300B,300C, 300D, 300E,300F, 300G, 300H, and 300I The L-shaped polycrystalline silicon layer300 intersect with the scanning line 320 and its I-shaped extensionportion by two regions. One is formed as the first channel 300C, and theother is the second channel 300G. As aforementioned in the firstpreferred embodiment, an insulating layer is formed before forming gatemetal layer 320. the regions abutting both sides of the first channel300C and the second channel 300G are n-type lightly doped regions 300B,300D and 300F, 300H, respectively. Still, the n+ heavily doped sourceregion 300A is contacted with the metal data line 330 through contactwindows 332. The n+ heavily doped drain region 300I is contacted withthe storage capacitor (not shown), the first interconnect line (notshown) and the transparent conductive electrode 360.

Similarly, the geometry of regions 300D, 300E, and 300F presented asright angle distributed is not necessary. It can be other geometry, forexample they can be presented as arc-shaped or a shortest line betweenthe channels 300 C and 300G. Furthermore, if the channel 300 c does notdistant from the channel 300G, for example, less than 1 μm, 1 n-lightlydoped region 300D formed is enough. Otherwise, it should have an extraheavily doped region to reduce resistance.

The fourth preferred embodiment is shown in FIG. 5, a top view. Thepolycrystalline silicon layer 400 formed on the glass substrate presentsas a L-shaped having regions including 400A, 400B, 400C, 400D, 400E,400F, and 400G. The L-shaped polycrystalline silicon layer 400intersects with the scanning line 420 and its L-mirror-shaped extensionportion 421 by two regions. One is formed as the first channel 400C andthe other is the second channel 400E. Basically, the fourth preferredembodiment is similar to the second preferred embodiment except, theextension portion of the scanning line is L-mirror-shaped in geometry.Thus, we will not recite them in detail.

The fifth preferred embodiment is shown in FIG. 6, a top view for apixel. The polycrystalline silicon layer 500 formed on the glasssubstrate presents as a L-shaped having regions including 500A, 500B,500C, 500D, 500E, 500F, 500G, 500H, and 500I The L-shapedpolycrystalline silicon layer 500 intersect with the scanning line 520and its L-shaped extension portion 521 by two regions. One is formed asthe first channel 500C, the other is the second channel 500G. Theregions abutting both sides of the first channel 500C and the secondchannel 500G are n-type lightly doped regions 500B, 500D, and 500F,500H, respectively. Still, the n+ heavily doped source region 500A iscontacted with the metal data line 530 through contact windows 532. Then+ heavily doped drain region 500I is contacted with the storagecapacitor (not shown), the first interconnect line (not shown) and thetransparent conductive electrode 560, as aforesaid in the firstpreferred embodiment.

The benefit of the present invention provided is:

The dual gate structure of the TFT according to the present invention atleast one gate channel is along data line direction. As a result, morepixels can be set along the scanning line and thus the pictureresolution can be elevated.

As is understood by a person skilled in the art, the foregoing preferredembodiments of the present invention are illustrated of the presentinvention rather than limiting of the present invention. It is intendedto cover various modifications and similar arrangements included withinthe spirit and scope of the appended claims, the scope of which shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar structure

1. A structure of a multi-gate thin film transistor (TFT) for LCD,comprising: a polycrystalline silicon layer formed on a substrate havinga source region, a first doped region, a first gate channel, a seconddoped region, a second gate channel, a third doped region, and a drainregion, further, said polycrystalline silicon layer having a firsthorizontal section formed said source region at an end thereof and afirst vertical section formed said drain region at an opposite endthereof; a multi-gate, including a scanning line and an extensionportion in every pixel formed on said substrate, said extension portionin a shape of L mirrored so that said extension portion with a parallelportion toward a source line and in parallel with said scan line and avertical portion connected with said scan line, and said scanning lineand said parallel portion intersected said first vertical section ofsaid polycrystalline silicon layer to form said first gate channel andsaid second gate channel, respectively; and a gate oxide layer formed inbetween said multi-gate and said polycrystalline silicon layer.
 2. Thestructure of a multi-gate thin film transistor (TFT) for LCD accordingto claim 1 wherein said first doped region, said second doped region andsaid third doped region are impurity lightly doped regions.
 3. Thestructure of a multi-gate thin film transistor (TFT) for LCD accordingto claim 1 wherein said second doped region further comprises animpurity heavily doped region in mid of said second doped region.
 4. Astructure of a multi-gate thin film transistor (TFT) for LCD,comprising: a polycrystalline silicon layer having a first horizontalsection and a first vertical section, wherein the first horizontalsection is parallel with a scanning line and the first vertical sectionis parallel with a data line; a multi-gate, including said scanning lineand an extension portion in every pixel, said extension portion in ashape of L mirrored so that said extension portion with a parallelportion toward said data line and in parallel with said scan line and avertical portion connected with said scan line, and said scanning lineand said parallel portion intersected said first vertical section ofsaid polycrystalline silicon layer to form a first gate channel and asecond gate channel, respectively; and a gate oxide layer formed inbetween said multi-gate and said polycrystalline silicon layer.